Fiber optic sensors are devices which measure some physical quantity using a modulated optical signal. Generally, in a fiber optic sensor an optical signal is transmitted through an optical fiber to a transducer which modulates the optical signal in a manner corresponding to the condition being monitored. The modulated light is then transmitted through a return optical fiber for detection so that the detected signal can be used to display the condition of the physical parameter being measured and/or for use in apparatus or process control.
A special class of the fiber optic based sensor is the two-state fiber optic sensor used for monitoring a parameter which may only be in one of two states. By way of example, such sensors include fiber optic based switches which can be either ON or OFF, such as fluid sensors which indicate the presence or absence of fluid at a particular sensing location. One such fluid sensor is disclosed in co-pending U.S. patent application Ser. No. 06/915,159 filed on Oct. 3, 1986 for FIBER OPTIC FLUID SENSOR.
Two-state fiber optic sensors operate by comparing the amount of optical power received by a detector to a preset threshold value. If the detector signal corresponding to the light received is above the threshold value the sensor output is set to one state, whereas if the detector signal is below the threshold value the sensor output is set to the other state. For simplicity, the state where the return signal is above the threshold value may be referred to as the "ON" state and the state where the return signal is below the threshold value may be referred to as the "OFF" state. By way of example, in a fluid sensor the presence of the fluid at a particular sensor location might create an ON condition signal above the threshold value whereas the absence of the fluid might give rise to a detector signal below the threshold value, creating an OFF condition.
Unfortunately, however, two-state fiber optic sensors cannot distinguish an OFF return signal from a sensor system failure, e.g. where the light source burns out or a fiber optic link breaks or becomes disconnected. It has heretofore been known that a small amount of error detection can be obtained by adding a second optical channel devoted to monitoring the state of the sensor system. This approach is of limited value. First, a failure in the error detection channel is not necessarily indicative of a failure in the sensing channel and vice versa. Second, adding a second sensor channel to the sensor system undesirably increases the cost, size and weight of the sensor.
Two-state fiber optic sensors also suffer from gradual system degradation short of system failure which may nonetheless result in erroneous ON-OFF sensor indications. Such a gradual failure can, for example, result from decreased optical efficiency caused by an accumulation of dirt on electro-optical surfaces or other factors resulting in degradation of electro-optical elements. A dedicated error detection channel is not useful for monitoring gradual system failure and is itself subject to this very limitation.
Therefore, it is one object of the present invention to provide an error detection system.
It is a further object of the present invention to provide an error detection system which is particularly useful with a two-state fiber optic sensor.
Another object of the present invention is to provide, in a two-state fiber optic sensor, an error detection system capable of recognizing a total sensor system failure.
Yet another object of the present invention is to provide, in a two-state fiber optic sensor, an error detection system capable of recognizing a change in sensor state caused by gradual sensor degradation.
It is yet a further object of the present invention to provide an error detection system capable of recognizing total sensor failure or gradual sensor degradation without adding appreciably to the cost, size or weight of the sensor.
These and other highly desirable and unusual results are accomplished by the present invention in an economical structure which accurately determines whether either total or gradual system failure has occurred.
Objects and advantages of the invention are set forth in part herein and in part will be obvious herefrom, or may be learned by practice with the invention, the same being realized and attained by means of the instrumentalities and combinations pointed out in the appended claims.
The invention consists in the novel parts, constructions, arrangements, combinations, steps, and improvements herein shown and described.